If you want to alert your colleague/client about task or subtask updates, set up following of the sub/task. The user gets notifications about all changes within the sub/task (new comments, completion, changed due date, completed subtask etc.).
Go to sub/task detail and scroll down, below the comment form. There is section Followers. You see current users, who get notifications about task changes.
Click person icon (Add/Remove followers…) and there you can add or remove users from following. Every user can change followers.
Check or uncheck the boxes and confirm your changes via Save button. This can be changed without adding a new comment.
Another way is useful for one time notifications. Do you want to get opinion or just notice someone else about the status?
We would recommend using mentions. Type a comment and mention someone else via @UserName or mention the whole project via @project. These users get one time notification. They do not become tasks followers.